The invention relates generally to the control of electric motors. Particularly, this invention relates to electric motor braking using regenerative braking or direct current (DC) injection braking or a combination thereof.
Electric motors, as used in many applications, typically require braking during their operation. For example, electric motors may be used in certain propulsion systems as prime movers, such as for vehicle drives. Further, during the course of motion of a vehicle or other inertial load it may be desirable to slow the load or bring it to a complete halt. More generally, braking systems and methods are an integral part of any system utilizing electric motors whose speeds may vary according to certain operational requirements. Many motors, particularly those driven at variable speeds, are driven by inverters, and braking may provide energy to a DC bus that provides power to the inverters.
Methods of motor braking may utilize the motor itself or, alternately, utilize a system of motors as part of a braking mechanism. Accordingly, motor braking techniques may include regenerative braking, whereby a motor generates a braking torque under its inertial loading. For example, in case of induction motors, a motor is supplied with current at a frequency that is less than a rotational frequency, i.e., speed of the motor, resulting in regenerative braking and in energy supplied to a DC bus coupled to the inverter feeding the motor. In another approach, energy provided by such inertial loading can be dissipated by resistive elements connected to the dc bus, disposed in the motor, or can be used for motoring another motor, if available. However, there are systems where this may not be possible and, consequently, the inertial loading of the motor may saturate the DC bus of a driving inverter with a high voltage, causing the DC bus to trip. Under such circumstances, the motor ceases to supply a braking torque to the system to which the motor is coupled, consequently, curtailing system stoppage.
There is a need in the art for improved techniques for braking inertial loads. In particular, there is a need for an approach to braking such loads that provides for using motors for braking purposes, while avoiding overloading an inverter DC bus in a way that causes overvoltage conditions on the bus.